Technical Field
The present invention relates to a probe card for a testing apparatus of electronic devices.
The invention relates to, particularly, though not exclusively, a high-planarity probe card and the following description has been made with reference to this field of application with the only purpose of simplifying the presentation.
Discussion of the Related Art
As it is well known, a probe card is a device suitable for placing a plurality of contact pads of a microstructure, particularly an electronic device integrated on a wafer, into electrical contact with corresponding channels of a testing apparatus performing the working test thereof, particularly an electrical test.
The test, which is carried out on integrated circuits, is particularly used for detecting and isolating defective devices already in the manufacturing phase. Generally, probe cards are used for electrically testing integrated circuits on wafers before cutting (singling) and mounting them inside a chip package.
A probe card comprises a testing head including, in turn, a plurality of movable contact elements or contact probes, which are provided with at least one portion or contact tip for a corresponding plurality of contact pads of a device under test. With the terms end or tip is meant herein and below an end portion, which is not necessarily pointed.
Therefore, the effectiveness and reliability of a measuring test, among other factors, is known to depend also on the creation of a good electrical connection between a device and a testing apparatus, and therefore on the establishment of an optimal probe/pad electrical contact.
Among the types of testing heads used in the technical sector considered herein for the integrated circuit testing, the so-called testing heads with cantilever probes are widely used, which have probes protruding like a fishing rod over a device under test and are also called cantilever testing heads.
Particularly, a cantilever testing head of a known type usually supports a plurality of flexible, in general threadlike, probes with preset electrical and mechanical properties. The probes, cantilevered from the cantilever testing head, have a substantially hooked shape, for the presence of an ending segment substantially elbow-folded having a generally obtuse internal angle.
The good connection between probes of a cantilever testing head and contact pads of a device under test is ensured by pushing the testing head on the device itself, the probes undergoing thereupon a vertical bending (relative to the plane defined by the device under test) in an opposite direction to the movement of the device towards the testing head.
The hooked shape of the probes is such that the probe contact tips, during the contact with the contact pads of the device under test and during the probes excursion upwardly beyond a preset contact point usually called “overtravel”, slide on the contact pads over a length usually called “scrub”.
Testing heads with vertical probes, which are designated as “vertical probe heads”, are also known in the art. A vertical probe head comprises a plurality of contact probes held by at least one pair of plates or dies, which are substantially plate-shaped and parallel to each other. These dies are provided with specific holes and are arranged at a certain distance from each other such as to leave a free area or air gap for the movement and possible deformation of the contact probes. The pair of dies particularly comprises an upper die and a lower die, both being provided with guiding holes in which the contact probes, generally formed by special alloy wires having good electrical and mechanical properties, slide axially.
Also in this case, the good connection between the testing probes and contact pads of the device under test is ensured by pressing the testing head on the device itself, with the contact probes movable within the guiding holes provided in the upper and lower dies being caused to bend within the air gap between the two dies, and to slide within these guiding holes upon this pressing contact.
Moreover, the bending of the contact probes within the air gap can be facilitated by a suitable configuration of the probes themselves or the dies thereof, particularly by either using contact pre-shaped probes or conveniently horizontally shifting the plates comprising them.
Testing heads are generally used, which have probes that are not fixedly fastened but hold interfaced to a card, which is connected in turn to the testing apparatus: these are called testing heads with unlocked probes.
In this case, the contact probes also have an end or contact head towards a plurality of contact pads of that card. The good electrical contact between the probes and the card is secured similarly to the contact with the device under test by pressing the probes onto the card contact pads.
Moreover, the card is generally held in position by means of a stiffener. The assembly of testing head, card and stiffener forms a probe card, which generally and schematically designated with 10 in FIG. 1.
Particularly, the probe card 10 comprises a testing head 1, which in the example of the figure is a vertical probe head. In this case, such a testing head 1 comprises, in turn, at least one upper plate or die 2 and a lower plate or die 3, having respective upper and lower guiding holes inside which a plurality of contact probes 4 slide.
Each contact probe 4 has at least one end or contact tip abutting onto a contact pad of a device under test 5, thereby providing the mechanical and electrical contact between said device and a testing apparatus (not illustrated) of which this testing head 1 forms a terminal element.
Moreover, each contact probe 4 has a further contact tip, which is designated as the contact head in practice, towards a plurality of contact pads of the card 6. The good electrical contact between the probes and the card is ensured similarly to the contact with the device under test by pushing the probes onto the contact pads of the card 6.
As stated above, the card 6 is held in position by means of a stiffener 8.
In the vertical probe technology, it is thus also important to ensure a good connection between the contact probes and the testing apparatus, particularly at contact heads thereof, and accordingly at the card.
Several technologies are known to make the probe card 10 of the testing apparatus.
Particularly, a first solution provides using printed circuit boards techniques to make the card 10, which is usually also designated with PCB (acronym for: “Printed Circuit Board”). This technology allows making cards having active areas that may even be large-sized, but have huge limitations relative to a minimum pitch value that can be achieved between the contact pads on the device under test.
The ceramic base technology or MLC (acronym for “Multi-Layer Ceramic”) is also known. Such a technology allows obtaining very fine pitches and higher densities as compared with PCB technology, however it has limitations on the maximum number of signals that can be used for the test and on the maximum dimension of the active area that can be placed on the card.
In the MLC technology, plates can be made of rigid ceramic material with a high planarity grade. These plates are not subjected to deformations or locally applied forces, which would only risk breaking them, but they can be provided with a central screw acting as a pivot and lateral adjustment screws associated with counter springs to allow a tilting of the plane defined by the plate itself.
Finally, a so-called hybrid technology can be used, where the testing head is interfaced to an intermediate plate, usually designated as interposer. This technology offers great flexibility in terms of surface, pitch and signal density, but is limited in the maximum number of manageable signals, having worst electromagnetic performances as well. A non-negligible disadvantage of the hybrid technology is that it is difficult to automatize.
US patent application published under No. US 2008/0157790 discloses a stiffener assembly, comprising an outer and inner member, to be coupled to an intermediate plate, wherein a plurality of alignment mechanisms allows the regulation of the inner member with respect to the outer member and allows a planar and/or lateral orientation of the probe head. The inner member is made of a material so as to ensure the rigidity of the probe card.
It should be understood that the thickness of the intermediate plate (interposer) is generally very low, ranging between 0.5 and 3 mm and therefore such plate has significant planarity problems. By being associated with the stiffener, it makes the whole assembly more rigid and resistant and allows a partial reduction in the defects of planarity in the intermediate plate, which however often result to affect the good operation of the cards made with this technology.